1. Field of the Invention
This invention relates to a valve assembly and, more particularly, to a valve seat and seal arrangement particularly suitable for plug valves operating under very high pressures.
2. Description of the Prior Art
Valves such as plug valves have long been popular as relatively inexpensive, simple and easily maintained means for controlling the flow of fluids. As these valves have been applied to systems handling high pressure fluids, various sealing problems have developed. The present invention relates to the sealing problems at the interface of the rotatable member found in such valves and the valve seats.
Prior attempts to seal the interface between the rotatable member and seats as found in plug valves have included careful machining of mating faces on each component which were then lapped together. Another system was to provide an "O" ring which was inserted in an annular groove provided in the face of the seat in concentric relation with the seat port. The careful machining technique is, of course, extremely costly, but does provide a reasonably good seal at least initially. However, it is most difficult to avoid at least some leakage under high pressures and the rotatable member can be difficult to rotate under very high pressures. Moreover, subsequent use in the presence of corrosive or erosive liquids quickly destroys the careful workmanship that has gone into the valve assembly, thereby destroying any seal obtained by the close tolerances.
The "O" ring insert has not been satisfactory because the rotation of the valve member from open to closed position of the valve or vice versa, causes the port in such rotatable member to pass over a portion of the periphery of the "O" ring, which expands into the port of such member and is then subjected to a shearing action which tends to pull the "O" ring from its seat or to destroy its sealing capabilities by a galling action. Even with this type of sealing arrangement, the costly machining technique could not be avoided as the principal sealing was still at the interface of the seat face and the rotatable member.
As an example of a more advanced attempt at a solution to the above problems, there may be mentioned Sachnik, Canadian Patent 690,218 issued July 7, 1964. It was the primary object of that patent to provide in combination with a through ported cylindrical plug valve and its enclosed ported housing, a valve seat structure which would resiliently support and constantly seal the cylindrical surface of the valve for such yielding thereof between the ports of the housing as became necessary under changes in pressures thereon under rotation, but also preventing expansion of any portions thereof into the ports of the valve and thus preventing any shearing or galling action as in the prior art structures. This involved the use of tubular valve seat members having concave end faces protruding into the valve chamber with the concaved face portions being recessed and retaining a resilient sealing ring bonded into the recess. While this system functions reasonably well under moderate pressures, the resilient sealing ring is unable to withstand the effects of very high pressures.
Part of the problem with the prior valve is that the sealing rings are serving not only a sealing function but also serve as valve seats for the positioning of the rotatable member. With such arrangement there is a substantial portion of the sealing ring which is actually exposed to the liquids under pressure and these become subject to cold flow. The result is deformation of the sealing rings with consequent leaking of the valve. In keeping with this, the patent mentions that under high pressures, the sealing ring is made of brass or bronze. Of course, when it becomes necessary to use a metal ring of this type, one is once more in the position of relying entirely upon accurate machining to provide the seal. This once more leaves open the problem of corrosive or erosive fluids under high pressure quickly destroying the careful workmanship that has gone into the manufacture of the valve.
Another valve seating assembly which has been developed to try to overcome the problems of high pressures is described in Shand, Canadian Patent 704,540 issued Feb. 23, 1965. This described a resilient annular valve seat which is pressed against the rotatable member by means of a spring. A gap is provided adjacent the resilient ring to permit some rocking thereof when the rotatable member is moved. This arrangement once again is not protected from the problems of cold flow of the plastic under high pressures and eventual leakage about the seal.
It is the object of the present invention to provide a valve assembly for use at high pressures which will avoid the difficulties inherent in the prior valves and which will permit easy turning of the rotatable member while avoiding leakage about the seals under high pressures.